Biological NMR spectroscopy has reached an exciting stage. (i) Structures of small proteins can be solved rapidly with current technologies. In fact, the collaborators on this PO1 grant have determined 28 structures of systems up to 35 kDa over the last five years, many of them supported by this grant. (ii) New NMR methods have emerged that have only partially utilized but promised a dramatic boost of speed, quality and quantity of structure studies. (iii) New hardware has arrived that can increase the sensitivity of spectrometers about fourfold, an unprecedented "quantum leap" which will make it possible to tackle large and difficult structural problems of high significance. At this point, NMR is not so much limited by technology; the main challenge is producing, in a useable form, biological samples that we want to study for biological reasons. Thus, the goals of the proposed research are three fold. First, continue to improve NMR and computational methods. Second, improve methods for expressing difficult isotope-labeled proteins in large quantities suitable for NMR experiments. Third, this effort will be shared and applied to studies of large systems containing DNA repair proteins, and as a challenging and medically important system, we will pursue aq structural study of the bacterial EntF protein of 142 kDa. The collaborators on this grant provide the expertise in NMR and computational methods, and in the production of biological molecules. They have a long history of collaboration and their group members share the laboratories whenever needed. Infrastructure support will be provided by three Cores. Core A (Administration), to be managed by G. Wagner, will provide administrative support to all participants. Core B (NMR Technology), also managed by G. Wagner, will make recent developments in NMR technology available for use in the above projects. Core C (Computation and Networking), will provide expertise state-of-the-art computing and networking environment.